1. Field of the Invention
The invention relates to a procedure for the production of thin-walled parts made of steel, which show an inner core layer and outer boundary layers. These layers are, formed based on their strength and hardness qualities, and at least partly differently treatable during a process. Furthermore, the invention relates to thin-walled parts made of steel with an inner core layer and outer boundary layers.
Thin-walled parts made of steel with a wall thickness of less than 4 mm, have a particularly high stress resistance which is demanded. This type of stress is for instance in mechanical engineering and vehicle engineering, wherein these parts are first thermoformed and/or cold-coiled, machined by metal-cutting or non metal-cutting and then tempered by a thermal treatment, namely tempered martensitic or bainitic. Out of hardened steel a part arises with continuous, uniform, high hardness along the complete cross section which has a low toughness. A more favourable combination of wear resistant surfaces with high toughness in the inner zone is achieved by the use of case hardened steels. This result can occur by a carbonizing treatment in a thermochemical hardening process wherein tempered, hard surface layers are produced while furthermore the inner core keeps a high toughness. This production procedure requires great effort and contrasts with the advantageous use qualities, however. A distortion of hardening is namely unavoidable through the relatively long case-hardening time of for example 180 minutes at 850-950 degrees Celsius and the following quenching in the oil bath or in the gas current. This process causes measure and form deviations which require an effortful subsequent treatment which quite considerably increases the production effort and expense. In addition, there is a relatively rough hardness structure which has an austenitic grain size according to DIN 50601 of for example 5 or 6. There is also a tendency towards intercrystalline failures which arise from the intercrystalline grain boundaries.
2. The References
As a substitute for the case hardening, furthermore the use of roll-bonded steel is known in which two or more, different alloyed tapes or panels get rolled together preferably from cold tape. By the pressure and the temperature, the core and surface layers of different alloyed steels are connected intimately with each other at the surfaces in the roll gap. A metallic compound arises from the following anneal by diffusion events. Such a roll bonding procedure is indicated in the reference DE 41 37 118 A1, for example. An abrupt, volatile changeover arises from it, however, between the different material layers. The hardness transition between layers that are tempered and not tempered is also therefore appropriately steep so that due to the load induced tension gradients, relatively thick surface layers must be produced. Through these relative tensions, the latent danger exists at the contact surface moreover unavoidably so that the peripheries chip off during use by transgression of the apparent yielding point in the joint area. This disadvantage can, as mentioned above, merely be met by surface layers being dimensioned more thickly. This result however, in turn leads to an unwanted higher wall thickness of the parts and moreover makes the production more difficult. The reference DE 196 31 999 A1 has already been suggested certifiable for the production of composite sheet metals in a continuous casting installation by casting together core and surface layers. Through this process, a steel layer material shall be produced. The difficulties at the production of layers which are differently tempered or hardened aren't taken up, however.
A similar continuous casting procedure is mentioned in the reference DE 33 46 391 A1 at which layer sheet metals are also embedded in a melt. The difficulties in the formation of layers which are differently tempered or hardened also aren't mentioned. The aforementioned continuous casting procedures and installations are obviously moreover suitable for only the production of relatively thick blanks or sheet metals and not for the production of thin-walled parts. It similarly behaves with U.S. Pat. No. 3,457,984 for the resulting level for the technological development. This reference refers, to or covers the casting rope of a continuous casting installation with sheet metal. Procedures and thin metal straps produced accordingly to the procedures are known of the reference DE-A-195 15 007 and the reference DE-A-198-50 213, in which a core material in layers of an economical material are spilled with thin metal straps to a composite material by a casting process, in which the thin metal straps are of use for the production of corrosion resistant and/or particularly smoother outer layers. A following treatment is not mentioned with respect to the influencing of other qualities of the composite material also here.